Fiber reinforced composite materials made up of reinforcing fibers and matrix resins allow material designs that make use of advantages of the reinforcing fibers and the matrix resins so that their use is being expanded to not only the aerospace field but also the field of sports, the field of general industry and the like.
As reinforcing fibers, glass fiber, aramid fiber, carbon fiber, boron fiber are used. Furthermore, as matrix resins, both thermosetting resins and thermoplastic resins are used, but thermosetting resins that can easily impregnate the reinforcing fiber are often used. As thermosetting resins, epoxy resins, unsaturated polyester resins, vinyl ester resins, phenol resin, bis-maleimide resins and the like are used.
For production of fiber reinforced composite materials, methods such as a prepreg process, a hand layup process, a filament winding process, a pultrusion process, and a RTM (resin transfer molding) process, are applied.
In recent years, as environment regulations for motor vehicles have been becoming stricter worldwide, domestic and foreign motor vehicle makers have been working on weight reduction of vehicle bodies, which influences fuel economy performance. In particular, application of carbon fiber composite materials, whose masses are half that of iron and about 70 percent of that of aluminum, is being actively considered. Various component members for motor vehicles are required to have lighter weights and also high rigidity and strength characteristics, and often have three-dimensional complicated shapes.
Therefore, the RTM process, which uses a high-rigidity and high-strength carbon fiber as a continuous fiber and is capable of coping with complicated shapes, is an effective forming method. The RTM process is a method of obtaining a fiber reinforced composite material by closing an upper mold after a substrate made of reinforcing fiber is disposed, injecting a matrix resin through a resin injection opening to impregnate the reinforcing fiber, then curing the resin, opening the upper mold, and extracting the molded product. A major issue for wider application of carbon fiber composite materials to motor vehicles is productivity. This poses an impediment such that carbon fiber composite materials have been only adopted in some high-class automobiles.
In the hand layup process, the filament winding process, the pultrusion process, and the RTM process, two-component type epoxy resin compositions are often used from the viewpoint of forming processability. The two-component type epoxy resin composition is an epoxy resin composition stored in a state in which a base compound liquid containing epoxy resin as a main component and a curing agent liquid containing a curing agent as a main component are put in separate containers and, immediately before use, the two liquids, the base compound liquid and the curing agent liquid, are mixed for use.
In contrast, an epoxy resin composition handled in a state in which all the components, including a base compound and a curing agent, are mixed in one is termed one-liquid type epoxy resin composition. In a one-liquid type epoxy resin composition, refrigerated storage is needed because a curing reaction progresses during storage. Furthermore, a less reactive solid curing agent is often selected and, to impregnate a reinforcing fiber with such a one-liquid type epoxy resin composition, the one-liquid type epoxy resin composition needs to be pushed in with high pressure by using a press roll or the like.
On the other hand, in the two-component type epoxy resin composition, since both a base compound liquid and a curing agent liquid are provided in liquid form, an epoxy resin composition obtained by mixing the base compound liquid and the curing agent liquid can be provided in a liquid form with low viscosity so that impregnation of reinforcing fiber is easy. Furthermore, since the base compound liquid and the curing agent liquid can be separately stored, long-term storage thereof is possible without any particular restriction on storage conditions.
To realize a high-level productivity as mentioned above, for example, in the RTM process, it is specifically required not only that the resin's curing time be short but also that four conditions (tasks) as follows be simultaneously satisfied. First, after being mixed and prepared, the epoxy resin composition shall be stable in viscosity for a long time with the viscosity being inhibited from increasing during storage at a low temperature of 40° C., that is, be excellent in the viscosity stability at 40° C. Second, in the step of injecting the resin into the reinforcing fiber substrate, the epoxy resin composition shall be low in viscosity and, during the injection step, increase in viscosity shall be inhibited and therefore the resin composition shall be excellent in impregnating ability. Third, sufficient high-speed curing can be achieved in an intermediate-temperature range around 120° C., thereby allowing simplification of the forming equipment and eliminating the need for heat resistance of subsidiary materials and the like. Furthermore, the resin itself shall not become colored so that both cost reduction and formed product quality can be favorably achieved. Fourth, in the mold release step after molding, the resin shall have attained sufficient rigidity due to curing and can be released from the mold smoothly without causing strain. Furthermore, strain or deformation will not result even from a painting step.
For these tasks, an epoxy resin composition (Japanese Unexamined Patent Publication (Kokai) No. HEI 6-256473) for optical uses that combines an acid anhydride as a curing agent and an ammonium organic acid salt as an accelerating agent and has intermediate-temperature rapid curability but no metal corrosiveness, an epoxy resin composition (Japanese Unexamined Patent Publication (Kokai) No. 2006-206862) in which an acid anhydride as a curing agent and a quaternary phosphonium salt as an accelerating agent are combined and the coloration during heated cure is inhibited, and an epoxy resin composition (Japanese Unexamined Patent Publication (Kokai) No. 2010-106131) that uses a non-aromatic based acid anhydride based curing agent as an alicyclic epoxy, and a salt of a quaternary phosphonium cation and an organic sulfonic acid anion as an accelerating agent and is excellent in ultraviolet radiation resistance and heat yellowing resistance are disclosed.
Furthermore, an epoxy resin composition excellent in the balance between the low-viscosity retention time and the curing time in a constant temperature condition around 100° C. due to use of an epoxy resin composition that combines an acid anhydride as a curing agent and an organophosphorus compound as an accelerating agent is disclosed (International Publication 2007/125759 pamphlet).
The materials described in Japanese Unexamined Patent Publication (Kokai) No. HEI 6-256473, Japanese Unexamined Patent Publication (Kokai) No. 2006-206862 and Japanese Unexamined Patent Publication (Kokai) No. 2010-106131 do not sufficiently achieve high-speed curing. The materials described in International Publication 2007/125759 pamphlet have a problem that the cured product becomes colored when molding is carried at high temperature to shorten the curing time.
Accordingly, it could be helpful to provide a two-component type epoxy resin composition that overcomes shortcomings of the foregoing conventional technology and is excellent in the viscosity stability at a low temperature (e.g., 40° C.) of an epoxy resin composition after the mixing preparation, retains low viscosity at the time of injection into reinforcing fiber and is excellent in impregnating property, cures in a short time during forming, and gives a fiber reinforced composite material high in the transparency of a cured product and excellent in formed product quality, and a fiber reinforced composite material made by using the two-component epoxy resin composition.